If you bare your teeth at a dog, it will feel threatened and attack you. If you do it to a monkey, it’ll treat it as a sign of submission. With people, however, it’s a little harder to say.

Smiling is different for everyone. I’ve photographed hundreds of people and I’ve seen them all – natural smiles, ecstatic smiles, timid smiles, fake smiles, awkward smiles, unwilling smiles… whatever the reason, whatever the reservation, they all smile for the camera.

Often they’ll mumble half a dozen rehearsed excuses in my direction while avoiding any eye contact; others freeze in a rictus grin as they wait for it all to be over. The majority of people I meet are uncomfortable with being photographed. Some are downright terrified. I’d say it’s right up there with public speaking.

So it’s weird. Not smiling in general, but the ritual of smiling for the camera.

I mean, the people in these photos aren’t actually happy – that much is pretty obvious. So what exactly are we capturing with these photos? A ‘moment’ for posterity? Well that’s clearly bullshit: it’s fake.

That might be the reason I enjoy casual photo shoots – being a fly on the wall, capturing spontaneous moments where people can relax and just be themselves. Most times they’re not even looking at the camera. But if they are, it’s natural. Comfortable. Like they’re glancing at a friend. Friends don’t always need to smile at each other.

That doesn’t work for everyone, though. Some people have a very particular idea of how they should appear in a photo – the angle, how wide the eyes are, the smile – and it’s a look they’ve no doubt practiced at home.

In fact, I’ve spied numerous women walking down the street snapping selfie after selfie on their smartphone, presumably for their Facebook profile or whatever… I’m pretty sure these are the girls with three hundred photos in Instagram with the same facial expression. I mean, doesn’t anyone else notice this? I feel like I’m taking crazy pills.

Back in the late 1800s to the early 1900s, it was considered a faux pas to smile for a photo because it could be easily mistaken for a mental deficiency. These days, in parts of Asia, for instance, it is considered rude to smile without covering one’s mouth.

So when did it become normal to smile in a photo?

Part of me thinks it happened some time in the 1960s, when American capitalism taught us we’re entitled to happiness. That explains why everyone looked a little glum in the early part of the 1900s. My grandparents, for instance. Even my parents. The photos we’ve taken since then seem to play to the idea that every moment we capture should be a happy one. Or maybe everyone in my family is just naturally frowny – I know I am, at least.

Charles Darwin is said to have observed that blind babies smile for exactly the same reasons (happiness, obviously), and in exactly the same way as sighted babies. The non-verbal behaviours that accompany smiles are learned.

The next time you stare down the barrel of a camera, ask yourself why you’re smiling at it. Ask yourself what you think is being captured, and what you’d like to be captured. That’s the little rabbit hole my mind dives into every time I take a photo of someone.